The present invention relates generally to the field of asynchronous transfer mode (ATM) communications and, more particularly, to logical multicasting in an ATM switch fabric.
In an ATM communications system it is often desirable to establish a point-to-multipoint virtual connection (VC), which is known as xe2x80x9cmulticasting.xe2x80x9d For example, a caller at a location in New York may wish to set up a conference call with three other participants, each of whom are in Los Angeles. Rather than establishing three separate cross-country VCs, it is desirable, from a bandwidth perspective, to have a single cross-country VC for the call which is then xe2x80x9cmulticastxe2x80x9d to each participant from a switch point which is closer to the recipients"" respective locations.
A multicast may be either spatial or logical. A spatial multicast occurs when the recipients are located at different physical switch output ports from one another. A logical multicast occurs when the recipients are located at the same physical output port, for example, on a LAN. So, in our example, if the three recipients are located at several unconnected offices scattered throughout Los Angeles, the multicast would be spatial. If, on the other hand, the recipients are located at a single company with a LAN, the multicast would be logical, since the call would be directed to all three recipients from a single switch output port at the local telephone company central office which serves the LAN.
Presently, ATM switches typically have a spatial multicasting capability, and switches which perform both spatial and logical multicasting are available. MMC Networks, Inc., for example, manufactures and sells a switch with full multicasting capabilities. Such switches however, have a number of disadvantages. Fist, a significant amount of available memory in the switch lookup tables must be dedicated to logical multicasting. In addition, logically multicasting switches typically are larger and use significantly more power than switches that do not have logical multicasting capability. They are also much more expensive. As those skilled in the art are well aware, space, power and cost constraints are important considerations in the design of any communications system.
In accordance with the present invention, an ATM cell entering a switch fabric input port is routed to its destination output port and to a predetermined dedicated second output port. The second output port is connected by a feedback connection to a predetermined dedicated second input port. The incoming cell can then be either switched to the same destination output port or switched to both the same destination output port and the predetermined dedicated second output port, thereby providing the ability to execute a logical multicast with any number of leaves to the destination output port.
It is therefore an object of the present invention to provide a logical multicasting capability using the smaller, cheaper and less power-consuming switches which do not have a built-in logical multicasting function. It is a further object to do so without adding significant size, cost or power consumption to the switch fabric.
For a better understanding of the present invention, together with other and further objects, reference is made to the following description, taken in conjunction with the accompanying drawings and its scope will be pointed out in the appended claims.